Process for making vanadium carbide briquettes



.ton of vanadium carbide is then United States Patent Timothy W. MertoVanadium Corpo- N.Y., a corporation of ABSTRACT OF THE DISCLOSURE Thisinvention is a method for producing vanadium carbide useful as a sourceof vanadium in the manufacture of vanadium containing alloys. Thevanadium carbide is prepared by reducing vanadium pentoxide to vanadiumtetraoxide which is further reduced to vanadium oxycarbide, addingcarbon, briquetting, and vacuum reducing to form the product.

This invention relates to improvements in the manufacture ofvanadium-containing alloys and relates in particular to a new and novelprocess for making vanadium carbide addition materials.

Master alloys or addition alloys for providing the element vanadium tosteel in the molten state are conventionally made by reducing vanadiumpentoxide. Fused and crushed vanadium pentoxide is a commerciallyavailable, relatively pure V 0 that is obtained by the chemicaltreatment of vanadium bearing ores.

Although commercially available methods for reducing vanadium pentoxidefrequently involve the production of an addition alloy known asferrovanadium, vanadium carbide briquettes, also the product of vanadiumpentoxide reduction, are finding increasing use as a van-adium alloyaddition source.

The metal vanadium has a high affinity for oxygen and consequently aportion of the vanadium is lost through oxidation when an addition alloyis introduced into molten steel which invariably contains residualoxygen. For many applications the use of the vanadium carbide briquettesis preferred over ferrovanadium because vanadium recovery (percent ofvanadium in the addition that actually goes into the alloy) from the useof briquettes is higher. The reasons for the higher recoveries whenemploying vanadium carbide is attributed to the carbon which is thoughtto protect the vanadium from oxidation as the alloy goes into solutionby preferentially combining with the oxygen present.

Vanadium carbide (8287% vanadium and 8-15 carbon) may be produced byheating a mixture of vanadium pentoxide and powdered graphite in anelectric arc furnace until a semimolten bath is formed. The semimoltenbath is permitted to freeze and cool to room temperature while withinthe furnace crucible. The butdug out of the crucible, cleaned, crushed,and further cleaned by gravity separation to remove graphitic carbon.This involved procedure produces insufficient vanadium carbode for largecommercial use and is primarily employed to produce vanadium carbide forcutting tools.

Commercially available vanadium carbide is produced by direct reductionat high temperatures in substantial vacuums. This procedure requireselaborate vacuum furnace structures capable of withstanding hightemperatures for extended periods of time.

We have devised a method whereby fused and crushed vanadium pentoxidemay be converted into vanadium carbide briquettes far more efficientlyand economically than the prior known methods. Our method combines heattreatment in gaseous environments and in substantial vacuums in a mannerto avoid the undesirable features of the aforementioned methods.

In general, the present invention consists of the followmg steps:

Step 1.-Crushed and fused V 0 is subjected to a temperature of fromabout 1050 F. to 1150 F. in a gaseous hydrocarbon reducing atmospherefor sunicient time to reduce the pentoxide to vanadium tetraoxide;

Step 2.-The vanadium tetraoxide is subjected to a temperature of atleast 1900 F. in a gaseous hydrocarbon reducing atmosphere so as toconvert the tetraoxide to a vanadium oxycarbide;

Step. 3.The vanadium oxycarbide obtained from Step 2 is cooled toambient temperatures while maintaining the material during cooling in anonoxidizing environment;

Step 4.-The cooled vanadium oxycarbide is mixed with a carbon sourcematerial in amounts to effect a stoichiometric balance of carbon tooxygen Within the mixture as to render an excess carbon content whensubject to the reaction of Step 6 below which when combined with thevanadium alone consists of from about 8 to about 15% (by wt.) carbon.Preferably, the material is comminuted to a very fine particle sizeprior to Step 5 herebelow;

Step 5 .-Briquetting the mixture of Step 4;

Step 6.-Subjecting the briquettes to a temperature of from about 2500F.-2700 F. while continuously withdrawing the evolving gases until theevolution of gas substantially stops; and

Step 7.-Cooling the briquettes to substantially ambient temperatures ina nonoxidizing environment.

Our process is not dependent on any one of the aforementioned stepsalone but instead provides a superior vanadium carbide briquettedproduct more efiiciently than has been possible heretofore by employingthe aforementioned sequence of steps.

The hydrocarbon reducing atmosphere of Step 1 of our process may consistof natural gas, which, though composed largely of methane, consists of amixture of hydrocarbon gases including propane, butane, etc.Substantially, pure methane, ethane, propane, or butane gases may beemployed, however, we have found that a mixture of one or more of thesegases is preferred.

Time at temperature of Step 1 is the time required to reduce the V 0 toV 0 at the temperature and in the atmosphere involved. The optimum timeto effect a substantially complete reaction is generally from about60-90 minutes.

It is preferable, though not essential, to continue heating thetetraoxide from the 1050-1150 F. temperature of Step 1 to the 1900 F.temperature of Step 2 while maintaining substantially the samehydrocarbon reducing atmosphere. We have found it to be particularlyexpedient to perform both of these steps in the externally firedrotating retort type of furnaces when the material is heated toprogressively higher temperatures from the end of the furnace in whichit is introduced to the end where it is discharged. We preferablyposition two of these furnaces in tandem so that the furnace of Step 1discharges into the furnace of Step 2 and the charge is progressivelyheated to the higher temperatures.

The atmosphere of Step 2, or furnace 2 of the process, is the same asthat of Step 1 so that a single elongated externally fired rotatingretort furnace may be employed for both Steps 1 and 2.

The temperature of 1900 F. of Step 2 is the minimum practicaltemperature for accomplishing the desired reaction so that a highertemperature (up to the melting point of the charge) is preferred.Heating equipment 3 limitations render the temperature range of from1900 F.2100 F. the most practical.

The time at temperature for Step 2 varies as in Step 1 but generally atime of from 100 minutes to 180 minutes is sufficient to reduce the V toa vanadium oxycarbide (VC O where x=0.4 to 0.6 and y=0.4 to 0.8)

The reactions of Steps 1 and 2 can be expressed by the followingformulae:

We prefer that the vanadium oxycarbide product of Step 2 have a carboncontent of 14 to 16%. While the composition of natural gas may be suchas to assure this carbon content in the vanadium oxycarbide, it isfrequently desirable to add propane gas to the natural gas. An alternatemethod of assuring a sufficiently high carbon content is to add finelydivided carbon with the original V 0 in Step 1, or to add it to the V 0feed material for Step 2. Coke derived from natural gas has been foundeffective in raising the carbon content of the VCO with a consequentlowering of the oxygen content.

In Step 3 the vanadium oxycarbide may be cooled while maintaining thereducing atmosphere or while substituting some other nonoxidizingatmosphere.

In Step 4 the vanadium oxycarbide is mixed with a carbon source materialin preparation for the reduction of Step 6. The preferred ultimateproduct is a vanadium carbide that contains from about 23-13% carbon.Vanadium oxycarbide is a homogenous finite material that has a carboncontent that can vary from 6 to 22%, but usually varies from 12 to 16%.At the temperature and pressure levels available in commercial vacuumfurnaces, about carbon is considered to be an optimum equilibrium pointbetween carbon and oxygen so that in Step 4 there must be astoichiometric balance between the carbon present and the oxygen contentof the vanadium oxycarbide. The total carbon content of the mixture mustbe such that there is an excess of about 10% carbon over thestoichiometric amount required to combine with the oxygen present in thevanadium oxycarbide.

The carbon source material may be any of the well known carbon richmaterials that are disposed to readily yield carbon such as lamp black,graphite, etc.

The mixture must be thoroughly mixed and comminuted to very fineparticle size, preferably in a ball mill or rod mill. The comminutedproduct preferably has a particle size wherein a maximum of about isover 100-mesh and a minimum of 60% will pass through a 325-mesh screen.

In Step 5 the above mixture is agglomerated. Such agglomeration may beaccomplished by a great variety of compacting means whereinpredetermined quantities of the particulate mass are compressed in sucha manner as to effect bodies with suflicient green strength to enablethem to be handled for subsequent sintering operations. For example, themixture may be compacted in a roll type brique-tting press. The size ofthe briquettes produced may be as small as x x /8, but preferably areabout 1%" x 17 x 1" or larger.

In Step 6 the briquettes are heated to within the temperature range ofabout 2500 F. to 2700 F. in a conventional vacuum furnace. Evolvinggases (primarily C0) are drawn off the furnace as fast as possible bythe vacuum pumps and the pumps continue to run until the pressure in thefurnace drops to a value of less than about 0.050 mm. of mercury. Theheating of the furnace is then discontinued.

Incooling the sintered briquettes an inert gas, such as argon, ispreferably introduced into the vacuum furnace and the furnace is cooledto yield the desired vanadium carbide sintered product. The sinteredbriquettes may,

of course, be cooled in any appropriate nonoxidizing environment.

In practicing the method of the present invention, vanadium pentoxidepreviously fused and crushed to the size of 20-mesh and down was chargedto an externally heated rotating retort batch type calcining furnace. Aflow of natural gas was started through the furnace and the heating ofthe retort begun. The furnace was held at 1000 F. for a short timebefore continuing the heating to the maximum temperature. The originalpurpose of such a hold was to avoid fusion of the pentoxide. Uponreaching the temperature range of 1950" F. to 2050 F. the charge wassampled continuously from the gas exit port.

Analysis of the samples showed an increase in carbon and vanadiumcontents as the heating continued. When the vanadium reached to 71% andthe carbon content 14 to 16%, the heating was discontinued and thecharge allowed to cool in the furnace without exposure to air.

Later, the process was carried out in a continuous externallyfired'rotating retort calcining furnace. With this furnace, the processwas carried out in two steps; the fiirst, at about 1100 F. to reduce V 0to V 0 and the second step at about 2000 F. to reduce V 0 to VC O Thehigh production rate made possible by the use of the continuous furnacewith natural gas as a reducing agent is a significant part of thepresent invention.

The pre-reduced product described above was found to be superior to anyof the oxides, V 0 V 0 and V 0 as a starting material for the vacuumreduction step. Its higher vanadium content and lower oxygen con tentrequired significantly less removal of the carbon monoxide during thevacuum step. A greater number of vanadium units can be charged into agiven vacuum furnace using the VC O as compared with the use of any ofthe other vanadium oxides used as starting materials. The density ofbriquettes made with VC O is greater than that of briquettes made withvanadium oxide. Sintering of the briquettes during the vacuum reductioncycle to obtain a desirable dense product is facilitated by the use ofVC O as an intermediate product.

For commercial production, it has been found to be particularlydesirable to employ two externally fired rotating retort calciningfurnaces, the first being shorter than the second to accommodate theshorter dwell time for the first step as compared with the second step.The two furnaces are arranged in series so that the product of the firstfurnace is discharged directly into the second furnace, thus conservingsensible heat. For both furnaces, the feed rate of the charge, thevolume of the natural gas flowing countercurrent to the solid charge,the inclination of the rotating retort, the speed of rotation, and thetemperature of the furnace are adjusted so that the product dischargingfrom the furnace is of the desired composition. For the first step, theproduct contains from about 57 to 61% vanadium with less than 0.25%carbon, the balance being largely oxygen with about 1% iron and tracesof other impurities. The desired composition of the product from thecalcining furnace is as follows: Vanadium, 68-72%; carbon, 14-16%;oxygen, 1014%.

The oxycarbide of vanadium has been found to be a superior material forthe final vacuum reduction step. Prior to this step it is necessary toadd additional carbon in some pure form to the pre-reduced product. Asstated above, the amount added is stoichiometrically proportional to thecarbon and oxygen contents of the oxycarbides. After comminuting to afine particle size, then cornpacting to a convenient size, final vacuumreduction is carried out at a temperature in the range of 2500-2700 F.Heating is continued until the carbon-oxygen reaction ceases, that is,when the pressure in the vacuum furnace drops to a low value below 0.050mm. of Hg.

The oxycarbide product contains significantly less oxygen to be removedby the vacuum treatment than. any

of the vanadium oxides. This is illustrated in the follow- One object ofthe vacuum treatment is to remove the oxygen content of the briquettedmixture. This is done through the carbon-oxygen reaction at hightemperatures and low pressures to form carbon monoxide which isexhausted through the vacuum pumps. A convenient measure of the extentof this reaction is to determine the weight loss of the product. Forcomplete oxygen removal the following weight losses are shown for thedifferent starting materials, expressed as pounds of gas per ton ofcharged product. The larger the volume of gas to be removed, the longerthe cycle will be, or, the larger the pumping system must be.

Table II Nominal weight loss pounds per Starting material: ton ofcharged product vco 420 v 790 v 0. 950 v 0 1050 TABLE III Relativeweight Relative volume Density of I Starting charged of charged Chargedmaterial, Material material per material per pounds per pound of V poundof V cu. ft.

VCO 1 l 86 V203" 1. 22 1.72 61 V204 1.41 2. 24 54 V 05" 1. 78 3. 56 43Considering the relatively high cost of vacuum furnaces and the amountof depreciation which must be assigned to the cost of producing theproduct, the economic advantage of using vanadium oxycarbide as astarting material for vacuum reduction is obvious.

It is important that any additive for use in steelmaking have arelatively high density to aid in its solution to molten steel. Thedegree of sintering during the vacuum reduction determines the densityof such products. We have found that higher densities are obtained usingvanadium oxycarbide as a starting material for vacuum reduction ascompared with products using any of the oxides. Many trials were madeunder controlled laboratory conditions using optimum particle size ofstarting materials, optimum briquetting pressures and optimum vacuumtreatment conditions. It was found that significantly higher densitieswere obtained when using Vanadium oxycarbide as a starting material.This is illustrated in the following table:

Table IV Starting material: g gi gfi gg VCO 4.0-4.5 V 0 3.4-4.0 V 03.13.6 V 0 2.4-3.0

While we have shown and described the preferred embodiments of ourinvention, it may be otherwise embodied within the scope of thefollowing claims.

We claim:

1. A method for producing a compacted and sintered vanadium carbideaddition alloy from fused and crushed vanadium pentoxide comprising:

(a) subjecting said pentoxide to a temperature of from about 1050 F. to1150 F. in a gaseous hydrocarbon reducing atmosphere so as to convertsaid pentoxide to vanadium tetraoxide;

(b) subjecting said vanadium tetraoxide to a temperature of at least1900 F. in a gaseous hydrocarbon reducing atmosphere so as to convertsaid tetraoxide to vanadium oxycarbide;

(c) cooling said vanadium oxycarbide to substantially ambienttemperatures from said at least 1900 F. in a nonoxidizing environment;

((1) mixing a carbon source material with said vanadium oxycarbide inamounts to effect a stoichiometric balance of carbon to oxygen withinsaid mixture as to render in the alloy an excess carbon content whichwhen combined with the vanadium content alone consists of from about 8%to about 15% (by weight) carbon;

(e) briquetting said mixture;

(f) subjecting said briquettes to a temperature of from about 2500 F. to2700 F. while continuously withdrawing the evolving gases until theevolution of gas substantially stops; and

(g) cooling said briquettes to substantially ambient temperatures in anonoxidizing environment.

2. The method of claim 1 wherein said gaseous hydrocarbon reducingatmospheres of (a) and (b) are composed of the gaseous state of at leastone material selected from the group consisting of natural gas, methane,ethane, propane and butane.

3. The method of claim 1 wherein finely divided carbon is added to thevanadium pentoxide prior to Step (a).

4. The method of claim 1 wherein finely divided carbon is added to thevanadium tetraoxide prior to Step (b).

5. The method of claim 1 wherein said 1050 to 1150 F. treatment isconducted for a time of from 60 to minutes and said at least 1900 F.treatment is conducted for a time of from to minutes.

6. The method of claim 1 wherein said carbon source material is added inamounts to effect a stoichiometric balance of carbon to oxygen withinsaid mixture as to render in the alloy an excess carbon content whichwhen combined with the vanadium content alone consists of from about 8%to 13% (by weight) carbon.

7. The method of claim 1 wherein said mixture of carbon source materialand vanadium oxycarbide is comminuted to a particle size wherein amaximum of 15% is over IOO-mesh and a minimum of 60% will pass through a325-mesh screen.

8. The method of claim 1 wherein said briquettes are of a minimum sizeof about /3" x x /8".

9. The method of claim 1 wherein said 2500 F.2700 F. treatment isconducted within a vacuum furnace with the vacuum pumps running to drawolf all evolving gases, said treatment continuing until the pressure inthe furnace drops to a value of less than about 0.050 mm. of mercury.

10. A method for producing a compacted and sintered vanadium carbideaddition alloy from fused and crushed vanadium pentoxide comprising:

(a) heating said pentoxide to a temperature of from about 1050 F. to1150 F. in a gaseous hydrocarbon reducing atmosphere so as to convertsaid pentoxide to vanadium tetraoxide;

(b) heating said tetraoxide from said 1050 F. to 1150 F. temperature toa temperature of from about 1900 F. to 2100 F. while maintaining agaseous hydrocarbon reducing agent so as to convert said pentoxide to avanadium oxycarbide;

(c) cooling said oxycarbide to substantially ambient temperatures in anonoxidizing atmosphere;

((1) mixing a carbon source material with said vanadium oxycarbide instoichiometric amounts to react with the oxygen content of saidoxycarbide and in combination with the carbon present in said oxycarbideto render in the alloy a substantially oxygen free vanadium carbidecontaining from about 815% (by weight) carbon;

(e) comminuting said mixture to a particle size wherein a maximum of 15%is over 100-mesh and a minimum of 60% will pass through a 325-meshscreen;

(f) compressing said mixture into briquettes;

(g) heating said briquettes to within a temperature range of 2500 F. to2700 F. in a vacuum furnace with the vacuum pumps running to removeevolved gases and continuing said treatment until the pressure withinsaid furnace drops to a value of less than about 0.050 mm. of mercury;and

(h) cooling said briquettes in a nonoxidizing atmosphere.

11. The method of claim 10 wherein Steps (a) and (b) are conducted inin-line externally fired rotating retort furnaces wherein the furnaceaccomplishing Step (a) is disposed for discharge Within a nonoxidizingatmosphere into the furnace for accomplishing Step (b).

12. The method of claim 10 wherein Steps (a) and (b) are conducted in asingle externally fired rotating retort furnace.

References Cited UNITED STATES PATENTS 12/1944 Benner et al 23--2082/1963 Robb 23-208

1. A METHOD FOR PRODUCING A COMPACTED AND SINTERED VANADIUM CARBIDEADDITION ALLOY FROM FUSED AND CURSHED VANADIUM PENTOXIDE COMPRISING: (A)SUBJECTING SAID PENTOXIDE TO A TEMPERATURE OF FROM ABOUT 1050*F. TO1150*F. IN A GASEOUS HYDROCARBON REDUCING ATMOSPHERE SO AS TO CONVERTSAID PENTOXIDE TO VANADIUM TETRAOXIDE; (B) SUBJECTING SAID VANADIUMTETRAOXIDE TO A TEMPERATURE OF AT LEAST 1900*F. IN A GASEOUS HYDROCARBONREDUCING ATMOSPHERESO AS TO CONVERT SAID TETRAOXIDE TO VANADIUMOXYCARBIDE; (C) COOLING SAID VANADIUM OXYCARBIDE TO SUBSTANTIALLYAMBIENT TEMPERATURES FROM SAID AT LEAST 1900*F. IN A NONOXIDIZINGENVIRONMENT; (D) MIXING A CARBON SOURCE MATERIAL WITH SAID VANADIUMOXYCARBIDE IN AMOUNTS TO EFFECT A STOICHIOMETRIC BALANCE OF CARBON TOOXYGEN WITHIN SAID MIXTURE AS TO RENDER IN THE ALLOY AN EXCESS CARBONCONTENT WHICH WHEN COMBINED WITH THE VANADIUM CONTENT ALONE CONSISTS OFFROM ABOUT 8% TO ABOUT 15% (BY WEIGHT) CARBON; (E) BRIQUETTING SAIDMIXTURE; (F) SUBJECTING SAID BRIQUETTES TO A TEMPERATURE OF FROM ABOUT2500*F. TO 2700*F. WHILE CONTINUOUSLY WITHDRAWING THE EVOLVING GASESUNTIL THE EVOLUTION OF GAS SUBSTANTIALLY STOPS; AND (G) COOLING SAIDBRIQUETTES TO SUBSTANTIALLY AMBIENT TEMPERATURES IN A NONOXIDIZINGENVIRONMENT.